1. Field of the Invention
The present invention relates generally to an improved method and apparatus for producing sheeting having precision patterns of micro-prismatic elements formed therein and, more particularly, to an improved method and apparatus for continuously embossing the surface of a film or film laminate with a pattern of precisely formed cube-corner retroreflective elements.
2. Description of the Related Art
Cube-corner type reflectors have long been in use in such applications as pavement markers, automobile reflectors and retroreflectors for use in highway signage construction, for example. The phrases "cube-corner," or "trihedral" or "tetrahedron" are well recognized terms in the art for structure consisting of three mutually perpendicular faces, without regard to the size or shape of each face, or the optical axis of the element so provided. One early example of a cube-corner type reflector is disclosed in the patent to Stimson, U.S. Pat. No. 1,906,655, issued May 2, 1933.
In more recent times, cube-corner retroreflective elements have been used advantageously not only in pavement markers and automobile reflectors, but also in flexible retroreflective sheeting suitable for use in highway signage construction, for example. Retroreflective sheeting requires, among other things, a drastic reduction in the size of the cube-corner elements by comparison to the elements used typically in pavement markers and automobile reflectors. Cube-corner type reflectors, to retain their functionality of reflecting light back generally to its source, require that the three reflective faces be maintained flat and within several minutes of 90.degree. relative to each other. Spreads beyond this, or unevenness in the faces, results in significant light spread and a drop in intensity at the location desired. A more detailed description of the optics of cube and microcube structures are found in commonly assigned co-pending application U.S. Ser. No. 08/655,545 (as published in PCT case US97/08806), the disclosure of which is incorporated herein by reference.
For many years, it was suggested that cube-corner retroreflective sheeting could not be manufactured successfully using embossing techniques (e.g., Rowland, U.S. Pat. No. 3,684,348, column 5, lines 30-42). However, embossing techniques were perfected such that embossed microcube retroreflective sheeting is now readily available. An example of a successful method for embossing sheeting is disclosed in U.S. Pat. No. 4,601,861, issued to Pricone et al. and assigned to the common assignee herein, the disclosure of which is incorporated specifically herein by reference.
While the method and apparatus disclosed in the aforementioned Pricone et al. patent performs effectively in continuously producing high quality microcube retroreflective sheeting, a disadvantage of such a system is the time involved in forming the prismatic elements. Generally, such a system is only capable of producing the embossed film at a rate of no more than thirty lineal inches per minute. The principal time factor in this system is that required to heat the film to its glass temperature, to enable formation of the microprismatic elements. This requires multiple embossing machines if high volume production is desired. Consequent cost in terms of machine maintenance and floor space, for example, also is therefore required. Accordingly, it is desirable to provide a method and apparatus capable of increasing production capacity of microcube retroreflective sheeting over prior known technology. Further, it is desirable to provide such a method and apparatus which produces a high quality finished product. It is further desirable to provide such a method and apparatus which is practical and relatively inexpensive to use. Still further, it is desirable to provide for the high production of embossed sheeting or laminates formed with precise patterns of micro-prismatic cells which can serve functions other than in retroreflective sheeting.